Adding a new cat to the family can be a smooth experience or a rocky road. Behaviourists hear from some owners of multiple cats that it seems their cats are not getting along as well as they had hoped. In some cases the relationship between cats is simply chilly.
For the most part these cats live in relative peace except for the occasional squabble. Mismatched personalities (for example, a timid, middle-aged cat sharing a home with a young, rambunctious newcomer) are almost certain to result in conflict now and then. But rest easy, research has shown that argumentative cats will tend to get along better the longer they live together.
So if your cats eat, sleep and groom comfortably in the same room, even if they sometimes hiss at, chase or wrestle each other, they will probably continue to live well together over the years. On the other hand, if fighting suddenly becomes severe, it may be time to seek the help of a behaviourist.